1. Field of the Invention
The present invention pertains to cooling devices employing conveyor belts for use in a continuous cooling operation, and in particular to freezers having a tunnel enclosure.
2. Description of the Related Art
Continuous process freezers and coolers have been employed for a variety of different products and are extensively employed in various food industries. For example, freeze-dried coffee, processed fish and meat parts are chilled or frozen in tunnel apparatus, using a variety of different coolant media. Brine solutions have been employed in these applications for some time, but cryogenic coolants such as liquid cryogen are becoming increasingly popular. Tunnel enclosures offer advantages of economical space utilization, and operational flexibility, since different cooling zones can be installed within the apparatus, to provide a continuous treatment of product as it travels through the tunnel enclosure.
Brine coolants offer significant disadvantages for processing food products. For example, contact between the brine solution and the food product must usually be avoided, and the viscosities of brine solutions change greatly over typical operating temperature ranges, thus complicating the flow handling equipment needed to circulate the brine solution. Cryogen coolants such as liquid nitrogen spray and bath treatments do not suffer from these drawbacks, and consequently are receiving widespread acceptance throughout the food industry. Cryogenic cooling can be carried out with conventional, readily available equipment and the viscosities and other physical properties of cryogenic coolants are well behaved over their typical operating ranges. Further, cryogenic coolants such as carbon dioxide snow and liquid nitrogen can be applied directly to food products without risk of contamination or deterioration of the product quality. Thus, economical direct contact spray and bath treatments of food products are possible with cryogenic coolants.
In cryogenic freezer tunnels and in other types of enclosures employing cryogenic coolants, the atmosphere is circulated using a plurality of fans, and circulations within enclosures are optimized to achieve different objectives. A typical purpose of such enclosures is to treat product carried on an endless conveyor belt which occupies the majority of interior volume within an enclosure. In order to minimize disturbance of desirable flow currents set up within an enclosure, so-called foraminous or perforate belting has been employed to convey products through an enclosure. Significant improvements in cooling rates can be achieved for a wide variety of food products using perforate conveyor belts; however, there are a substantial number of food products which are unsuitable for use with perforate conveyor belts. Examples of such products are chocolate, soup, semi-liquid foods, coffee extract, dough products and very small size or crumbly protein foods, such as processed meat and fish. It is desirable that such products be cooled using direct contact cryogenic techniques, and advances are being sought in this field.